As we mentioned earlier, missiles have a path control system and an attitude
control system. Guidance systems are usually classified according to their path
control system, since many missiles use the same type of attitude control. The
type of attitude control used in the fleet is inertial. The following is a
discussion of the types of path control (guidance) in use in SMS missiles.

INERTIAL GUIDANCE.-An inertial guidance system is one that is designed to fly
a predetermined path. The missile is controlled by self-contained automatic
devices called accelerometers.

Accelerometers are inertial devices that measure accelerations. In missile
control, they measure the vertical, lateral, and longitudinal accelerations of
the controlled missile (fig. 1-19). Although there may not be contact between
the launching site and the missile after launch, the missile is able to make
corrections to its flight path with amazing precision.

During flight, unpredictable outside forces, such as wind, work on the
missile, causing changes in speed commands. These commands are transmitted to
the missile by varying the characteristics of the missile tracking or guidance
beam, or by the use of a separate radio uplink transmitter.

BEAM-RIDER GUIDANCE.-A beam-rider guidance system is a type of command
guidance in which the missile seeks out the center of a controlled directional
energy beam. Normally, this is a narrow radar beam. The missile's guidance
system receives information concerning the position of the missile within the
beam. It interprets the information and generates its own correction signals,
which keep the missile in the center of the beam. The fire control radar keeps
the beam pointed at the target and the missile "rides" the beam to the target.

Figure 1-20 illustrates a simple beam rider guidance system. As the beam
spreads out, it is more difficult for the missile to sense and remain in the
center of the beam. For this reason, the accuracy of the beam-rider decreases as
the range between the missile and the ship increases. If the target is crossing
(not heading directly at the firing ship), the missile must follow a continually
changing path. This may cause excessive maneuvering, which reduces the missile's
speed and range. Beam-riders, therefore, are effective against only short- and
medium-range incoming targets.

HOMING GUIDANCE.-Homing guidance systems control the path of the missile by
means of a device in the missile that detects and reacts to some distinguishing
feature of (or signal from) the target. This may be in the form of light, radio,
heat, sound waves, or even a magnetic field. The homing missiles use radar or RF
waves to locate the target while air-to-air missiles sometimes use infrared
(heat) waves.

Since the system tracks a characteristic of the target or energy reflecting
off the target, contact between the missile and target is established and
maintained. The missile derives guidance error signals based on its position
relative to the target. This makes homing the most accurate type of guidance
system, which is of great importance against moving air targets. Homing guidance
methods are normally divided into three types:, active homing, semi-active
homing, and passive homing (fig. 1-21).

Active Homing.-With active homing, the missile contains both a radar
transmitter and a receiver. The transmitter radiates RF energy in the direction
of the

target (fig. 1-21, view A). The RF energy strikes the target and is reflected
back to the missile. (This process is referred to as "illuminating the target.")
The missile seeker (receiving) antenna detects the reflected energy and provides
it as an input to the missile guidance system. The guidance system processes the
input, usually called the homing error signal, and develops target tracking and
missile control information. Missile control causes the missile to fly a desired
flight path.

The effective range of the missile transmitter is somewhat limited because of
its size (power output). For this reason, relatively long-range missiles, such
as HARPOON, do not switch to active guidance until after midcourse guidance has
positioned the missile so that the transmitter is within its effective range.

Semiactive Homing.-In a semiactive homing system, the target is illuminated
by a transmitter (an illuminator) on the launching site (fig. 1-21, view B). As
with active homing, the transmitted RF is reflected by the target and picked up
by the missile's receiver. The fact that the transmitter's size is not limited,
as with active homing, allows a much greater range.

The missile, throughout its flight, is between the target and the radar that
illuminates the target. It will receive radiation from the launching ship, as
well as reflections from the target. The missile must therefore have some means
of distinguishing between the two signals, so that it can home on the target
rather than on the launching ship. This can be done in several ways. For
example, a highly directional antenna may be mounted in the nose of the missile;
or the Doppler principle may be used to distinguish between the transmitter
signal and the target echoes. Since the missile is receding from the transmitter
and approaching the target, the echo signals will be of a higher frequency. Most
SMS missiles use both of these methods.

A drawback of this system is that the shipboard illumination is not free to
engage another target while the missile is in flight. STANDARD SM-1 and
SEA­SPARROW all use semi-active homing as their primary guidance; they do not
use midcourse guidance. The STANDARD SM-2 uses midcourse guidance, and then
semi-active homing only for terminal guidance. As a result, the SM-2 needs
illumination from the ship only for the last few seconds of flight.

Passive Homing.-Passive homing requires that the target be a source of
radiated energy (fig. 1-21, view C). Typical forms of energy used in passive
homing are heat, light, and RF energy. One of the most common uses of passive
homing is with air-to-air missiles that use heat-sensing devices. It is also
used with missiles that home on RF energy that originates at the target (ships,
aircraft, shore-based radar, and so forth). An example of this is the STANDARD
ARM (anti-radiation missile) used for both air-to-surface and surface-to-surface
engagements. An advantage of this type of homing is that the target cannot
detect an attack because the target is not illuminated.

Several missiles that normally use other homing methods (active or
semi-active) are capable of switching to the passive home-on-jamming (HOJ) mode
in a countermeasure environment. That is, if the target detects that it is being
illuminated by an active or semiactive guidance radar and initiates jamming (RF
interference), the missile will home on the jamming signal if it is unable to
maintain track on the reflected illumination signal.

Tracking Radar/Fire-Control Radar

Radar that provides continuous positional data is called tracking radar. Most
tracking radar systems used by the military are also called fire-control radars,
the two names being interchangeable. A fire-control tracking radar system
produces a very narrow, circular beam.